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More than a Pretty Finish

New technology, new assignments, more opportunity

By Robert B. AronsonSenior Editor

Grinding technology is responding to the challenges of higher productivity, harder alloys, precision finish, and cost reduction. The answers lie in a new generation of abrasives, grinding centers, and process improvements.

Sales of abrasive machining equipment are experiencing an upturn because of worn equipment, a brightening economy, improved equipment, and tax incentives.

At the same time, major manufacturers, such as auto and aerospace manufacturers are closing in-house manufacturing operations and sending more components to outside suppliers.

"I am amazed at how many companies, particularly the larger ones, have only recently begun to abandon their decades-old abrasives and grinding machines and take advantage of the latest grinding technology," comments Denis Fritz, vice president and CEO of Erwin Junker Machinery Inc. (Miamisburg, OH). He finds most major manufacturing countries are ahead of the US in accepting this technology. Europe is leading, and, of course, Japan and China are using it extensively. Even India has accepted it.

"Junker offers multifunction designs. In one design we use three shafts with each shaft carrying up to three wheels. In another case we essentially combined two machines on one base so that roughing and finishing can be done on the same base. This configuration offers a smaller footprint, less setup time and part handling, as well as lower cost."

As for processes, Fritz explains, "We don't favor form grinding. Instead we offer our single-point grinding. In this design a thin wheel is used much like a lathe's cutting tool to grind complex shapes. This eliminates the problems of a dressing wheel's shape and changing wheels to handle a single complex part."

Holroyd-Edgetek (Farmington, CT) is another company that has added versatility to its line of grinding equipment. Jim Carroll, vice president, notes that developments in superabrasive grinding on machines with multiple wheels have allowed them to replace some milling and turning operations. "In several cases we have eliminated a conventional turning operation with high-efficiency superabrasive turning, using the company's 'SATurn' machine. We replaced the turning with a rough-grinding operation followed by a finishing operation on the same machine, saving the customer a two-machine operation. The main problem with conventional turning is high tool wear, and tool replacement is a long, costly operation."

In another recent application, Holroyd-Edgetek integrated one of its Edgetek superabrasive machines with a Mitutoyo CMM in a package that is replacing seven conventional milling and grinding machines at a leading global aerospace company.

"The Holroyd/Mitutoyo package gives the customer the lean manufacturing benefits of single setup, and the ability to reduce process chains by performing all operations on one machine," explains Carroll. "Consequently, the overall manufacturing process is not only substantially quicker, but also simpler and more controllable. Added to these benefits is accelerated payback on the $750,000 package [$650,000 Edgetek + $100,000 Mitutoyo CMM]; tooling and cycle-time reductions are projected to pay off the capital cost of the machines in 12 months.

"In our designs we have several wheels mounted on an HSK spindle," says Carroll. "We group wheels for roughing, profile grinding, and finishing. For example, the three operations require a single setup and changes in grinding operation only require an indexing of the spindle.

"Advances in grinding machine design have made this possible. We use a more rigid, vibration-damping polymer concrete in the base and column of the machine, and we can offer up to five axes with our trunnion-style machines. The key feature in high-efficiency superabrasive grinding is using only straight oil. Despite the claims for water-based or synthetic coolants we have found that neat oil does a better job with superabrasives and, surprisingly, is more environmentally safe."

John Drake, CEO of Drake Manufacturing (Warren, OH) also notes the increasing acceptance of CBN as a replacement for conventional established abrasives. But he sees smaller shops more aggressively using it than the bigger industries. "Smaller shops are more open to innovation and don't have the inertia of larger companies. Our nation has a large inventory of older machines, and big-company managers are less likely to encourage their engineers to take risks. A helpful intermediate step is to go from aluminum-oxide wheels to seeded gel. This improves productivity, but doesn't entail the high cost of implementing CBN on an older machine.

"The key difference in wheel grit is the type of fracture occurring during grinding," Drake explains. "A new aluminum-oxide wheel starts with sharp grains, but the grits round-off and are eventually broken from the wheel. CBN wheels start sharp and stay sharp. When they microfracture, the grains retain sharp edges.

"Seeded gel has some characteristics of both and offers an intermediate improvement in productivity. With seeded gel, the wheel is a combination of aluminum oxide and a grown crystal that fractures much like CBN."

According to some sources, an approximation of the ratio of metal removed to grinding wheel consumed (G-ratio) is:

Aluminum oxide: 5 to 10

Seeded gel: 50 to 100

CBN: 2000 to 5000

"You need a long run to make CBN profitable," says Drake. "It can be a lights-out process with the right machine instrumentation. You also need to keep the worker out of the process."

The company offers a new grinding system, the GS:TE-LM that features linear motors and high static and dynamic stiffness. It offers high precison for smaller parts such as taps, wormgears, and ballscrews. "We have watched the linear motor technology develop and, until recently, they were not smooth enough for low-speed travel. Our machines need both a slow feed rate and a short, fast stroke of around 1200 ipm [30 m/min]." Accuracy is measured in tenths of a micron. The unit features a full 180º power helix cutting head driven by a 7.5-hp (5.5-kW) spindle.

Customers want more. Survivors of the recent downturn generally agree that customers are demanding more than just a machine or wheel. "They want the supplier to be an all-knowing, all-providing source," observes Bruce Hammond of Campbell Grinder Co. (Spring Lake, MI) a major supplier to the jet engine market. "Because of the reduction in workforce of our customers, chiefly those in the aerospace industry, there is now a lack of shop-level people and process engineers that have the time to keep current with newer abrasive-machining processes. They therefore need help in taking advantage of this newer technology. We have taken on that responsibility, and turnkey work makes up about one quarter of our business. Our company also offers a variety of support contracts."

This support might include periodic visits to check operations, debug systems, and generally try to enhance operations. In some cases the contract includes a guaranteed cycle-time improvement or reduction in part cost. "Some of our models take on total process responsibility, with the ability to turn, grind, mill, and inspect, allowing the customer to load the part only once," says Hammond. "These machines have had the ability for more than 10 years to change tools and coolant nozzles, reset coolant flow rates and pressures as required and run any abrasive to optimize operating conditions. Another goal is to minimize operator interaction. When you have a CBN wheel worth $25,000 you want to minimize handling. One crash gets rid of your profits.

"Campbell builds machines for parts ranging in size from a 4" work cube and up. In one case we supplied a machine to grind 8-m diam telescope mirrors."

Bryant Grinder (Springfield, VT) is back in the market with a line of redesigned units. One example is the Model B+ CNC precision internal grinder, which replaces their Model B. It is a linear-motor machine that can use superabrasives, and is designed for jobs such as grinding miniature ball bearing liners and other parts down to 0.40" (10-mm) diam. The X and Z travel range is 3 X 3" (76 X 76 mm) with an accuracy of 0.0002" (5 µm) and a 0.000004" (0.1 µm) resolution. The unit has hydrostatic slides and can be quickly changed between a radius and straight dresser.

Changes in product design have increased demand in some areas of manufacturing. The new emission requirements for diesel engines have forced some engine builders to make significant changes in production processing. Fritz of Junker Machinery explains that they are selling well to those who make crankshafts for diesel trucks and busses.

"We offer a machine that has two independent spindles. The user can grind two main bearings at a time, and then two pin bearings. This dramatically shortens cycle time. We use a CBN wheel operating at about 120 m/sec.

"Our machines are particularly suited for the 'assembled' camshaft. The older design begins with a cast iron blank with the cam lobes cast in place. It was then ground to the proper shape and finish. With the assembled shaft, cams are finished separately then slipped over a hollow shaft and attached. The advantages are less weight, because the shaft is now hollow. There is less material to be removed, and different materials can be used for the shaft and cams.

"Buyers are demanding closer access to equipment manufacturers," says Nelson Beaulieu, United Grinding Technologies (Miamisburg, OH). "They want to work more closely with the manufacturer for faster turnaround of technical information, so we are digging deeper into the customer's processes upstream and downstream of the primary operations we traditionally focused on. We have, therefore, become solution providers using an entirely new set of guidelines based on 'profit processing,' not just machinery manufacturers.

"Many companies, in an effort to 'run lean,' have either eliminated or reduced toolrooms and maintenance systems, and no longer are capable of making prototypes economically. Now, when the pressure is on to produce, they are short-handed. That's where we can help.

"Also, due to lower production volumes there are more frequent part changes. That means more integrated use of grinding wheels and tooling for multitasking and taking advantage of software changes in lieu of tooling changes or experience. With these orders, takt time gives way to flexibility, and is no longer the driving issue. Again, this is profit processing, as opposed to simple part processing. The key to implementing this technique is to gain a better understanding of the customers' changing production needs. It's then economically sensible to accommodate a certain amount of lag time for indexing and loading and unloading that was not previously acceptable with high-volume production.

"Some grinding machines can use a number of wheels mounted in toolchangers, or ganged wheels with common forms mounted on common spindles and indexed as needed. The benefit of premounted wheels is realized by rapid setup for new jobs and the potential for upstream and downstream processing such as polishing, putting in snap-ring grooves, or spline grinding and even deburring.

"Problems, such as accuracy lost once a wheel is released and regripped, are overcome by automatically dressing. However, the economics of this action are now the driver, not the takt time.

"One problem with the magazine, even though more kinds of wheels are available, is you have to redress the wheel each time it's indexed. This wastes time and abrasive.

"Dressing pressures can vary greatly, from a relatively low pressure of 50 lb/linear inch (222 N/linear mm) of profile to highs of over 1000 lb/linear inch (4.6 kN/linear mm) of profile. This depends upon whether you're using diamond dressing rolls, dressing disks, or crush rolls, as well as the need to use unidirectional or bidirectional techniques. Because a flexible platform must address the most adverse conditions, some grinding machine builders provide that design feature. Considering that high dressing pressures may be required, a grinding platform has to handle these demands and move in submicron increments repeatedly in order to optimize the use of conventional and superabrasives, not to measure micron-range component tolerances.

"Peel grind has a place with many parts, particularly with parts under the length of 150 mm, given that lateral feed rates of 2 - 3 mm/sec typically can be applied at a depth of 0.5 - 1.0 mm or more on a radius on parts with an overall diameter in the range of 10 - 12 mm.

"Profit processing using a combination of grinding technologies on a common platform allows the user to extract more profit out of reduced production, enhances the part quality, and reduces tooling costs. But once you are over a certain part length, it's more economic to use plunge grinding," Beaulieu concludes.

Shigiya USA (Wheeling, IL) has achieved its major success in supplying grinding equipment for niche markets within the auto industry. Their equipment includes both conventional CNC machines with aluminum oxide wheels and more advanced systems designed for CBN.

In many applications, their grinders are used in conjunction with hard-turning operations, particularly on high-precision transmission components. "We also do a lot of peel grinding using CBN in place of plunge grinding using our patented system for wheel positioning," says General Manager Harry Kirihara.

It's because of Shigiya's expertise with such parts that Kirihara sees a lot of potential in the growing popularity of the continuously variable transmission (CVT).

This is a design that has been used successfully with smaller engines (less than 2 liter), such as those in snowmobiles. Because of developments in drive-belt design and sensing capabilities, several companies are trying to scale up the CVT for larger engines.

The CVT is simply two pulleys connected by a drive belt. Diameters of the two pulleys can be changed to vary the power transmitted without changes in engine speed. The design promises fuel savings and lower emissions, plus it is easier to build and maintain. "Our equipment would do a good job on the large faces and complex shapes required by the CVT," Kirihara concludes.

Okamoto Corp. (Buffalo Grove, IL) offers its UPG (ultraprecision grinding) series with an accuracy of 0.000004" (0.0001 mm). The machines are designed for the nanotechnology, super-high-precision market. Key to this precison is a high-precision segmented slideway. Sensors adjust hydrostatic pressure to keep the same oil film thickness due to weight shifts.

According to Victor Truelsen, application engineer, the company has seen a "tremendous increase" in ID grinding applications for high-precison work, such as drawing dies and aerospace bearings. This work, he notes, is growing in the US and not going to China. "For once the advantages are in our favor. The Chinese currently cannot achieve these accuracies, shipping can introduce an unacceptable turnaround time, and there may be inaccuracies caused by temperature variations over time."

"A major challenge for our company is getting purchased components to meet demand," says Denny Rowe, director, Weldon Solutions (York, PA). "We have a significant backlog of orders, chiefly because buyers took advantage of the 50% tax credit last year, which is now 10%. A second push is the legislation to reduce pollutants from diesel engines. The redesign requires much greater precision and manufacturers need new grinding equipment to do that.

"Another trend in grinding is greater use of a programmable C axis. A single wheel's motion in and out is coordinated with part revolution. That gives the user the ability to grind shapes, not just round diameters. Without the C axis, parts had to be rough milled, and then finished on a grinder. Now the grinder can handle shapes such as triangular, square, or other non-round configurations. That opens a big market for grinders in punch and cam production.

"We offer some machines with linear motors, particularly cam grinders. In addition to giving better control, the linear motor eliminates parts that can be long-term service issues such as ballscrews, couplings, and bearing assemblies.

"Although all this automation can reduce the number of machine operators, we find that it often increases the number of higher-level jobs. Companies with this complex equipment need good programmers and high-level maintenance personnel.

"For some time there has been a battle between hard-turning and grinding. But the greater use of harder materials is giving us an edge. For every application that we have lost to hard-turning, we have added jobs involving harder to machine metals, ceramics, and carbide," he concludes.

"Customers are continually asking for new products; their main interest is in reducing total cost," explains Patrick Redington, corporate engineering manager, Saint-Gobain Abrasives (Worcester, MA). "The two biggest requests are to eliminate rework or scrap and increase productivity. To achieve these goals we have to look at the total grinding system, which includes the wheel, machine, coolant, and the grinding process operating parameters. In all cases we offer engineering support with our abrasive products."

Despite the increasing demand for superabrasives, aluminum oxide, along with its many variations, is still the most commonly used abrasive. "One of the newest types is our Vortex Technology, which will be introduced shortly," says Redington. "It's basically a proprietary variation of aluminum oxide with a new bond system that gives longer wheel life, but requires less power for a given material removal rate (Q' in.3/min/in.). In recent customer testing it has worked extremely well on heat-sensitive material where metallurgical damage is an issue. It has a unique advantage for grinding heat-sensitive products such as aerospace components, automotive parts, gears, or any applications that have a large area of contact.

Another variation of aluminum oxide is seeded gel (SG). This abrasive is manufactured by sintering rather than fused, and the result is a grit with a different microstructure. Its "stay sharp" fracture properties approach those of CBN. So for some time it has been a lower cost substitute for CBN-type performance.

"When selecting an abrasive product, there is no one answer," says Redington. "The user has to select his grinding system, which includes the wheel, to meet his current and future production requirements. For example, in superabrasives alone you have diamond and CBN available in three main bond categories: plated, vitrified, and resin bonds. In general, plated wheels are easiest to use. They require no dressing, but must be discarded or replated after a fairly short production run. So they are best suited for small lot sizes. Resin-bonded wheels have an intermediate life. They require truing and dressing to enable the wheel to grind efficiently, and they also need a good quality grinding machine.

"Vitrified is the top performer for high-volume production. It has the longest life (G ratio), and needs to be trued but not dressed. It does need a high-performance grinding system to fully utilize this technology. It's often used in high-volume precision-grinding applications in the aerospace, bearing, and automotive industries," he explains.

This article was first published in the February 2005 edition of Manufacturing Engineering magazine.